In the ventral nerve photoreceptor of Limulus a short, intense flash evokes a receptor current consisting of three components. In contrast to other hypotheses, we suggested previously that only the second component of the current is activated by the phospholipase C pathway, which releases calcium from intracellular stores by inositol trisphosphate. The present paper gives further evidence to our suggestion. It is demonstrated that U-73122, a specific inhibitor for the phospholipase C, selectively blocks the second component. The first and third components are moderately affected and could still be activated after the complete block of the second one. Results support the idea that the first and third components areactivated by pathways operating independently of phospholipase C.

Normal photoreceptor cells on the ventral nerve of Limulus respond to a moderately intense flash with a large receptor potential or current. Occasionally, cells are found in which the same flash evokes only a small receptor potential or current. Our investigations reveal physiological reasons for the poor light sensitivity in these “unusual cells.” In unusual cells prolonged illumination with intense light evokes a step-like inward current with an amplitude of some nanoamperes, but without a large transient peak. The current appears to be summed up of single photon responses with amplitudes smaller than about 50 pA. Their time course is similar to that of small single photon responses forming the so-called macroscopic C1 component in normal cells. The macroscopic current evoked by an intense flash has slow activation and deactivation kinetics and reaches a saturated amplitude of about 4–5 nanoamperes. The light-intensity dependence of the current evoked by flashes or by prolonged illumination has a slope of about 1 in log–log plots. The decay kinetics of the current is similar to that of the C1 component measured in normal cells after the block of the C2 component. Occasionally, the step-like current is superposed by large standard bumps. These bumps are blocked by the Ca2+-ATPase inhibitor cyclopiazonic acid, while the sustained inward current persists. We conclude that in unusual cells the light-activated current is identical to the C1 component of normal cells. The phospholipase C pathway that in normal cells presumably gives rise to the C2 component functions only with a low efficiency in unusual cells.